The invention relates to a method and equipment for determining the longitudinal loading acting on a beam, such as on a rail of a railway track subjected to longitudinal loading and for determining more particularly the neutral temperature of railway tracks, by energizing the beam longitudinally in the audio-frequency range and measuring the level of Barkhausen noise at the surface of the energized region. Further subject of the invention is an equipment serving for implementing the proposed method.
In the elements used in the construction of track structures lower or higher residual stresses are present due to processes of manufacturing. Further mechanical stresses are added to these residual stresses by dead weight and installation. From the points of view of loadability and fatigue of structural parts, the overall resultant of all these stresses are to be taken into account. However, to find the balance of forces acting on a structure, the force arising in each individual element has to be determined.
The task of determining the forces acting in the various beams of a lattice truss or in a rail of a continuously welded railway track is not a simple technical problem. Application of destructive test methods is not expedient, because, for instance, in the case of a railway track such a test would require setting of scotch blocks.
In a welded track the sleepers prevent displacement of rails through the track fastening elements. After the rails have been clamped, any temperature change causes a thermal stress in the rails due to restriction of dilatation. The temperature at which the thermal stress in the tested cross-section of a rail is zero is termed neutral temperature. It is important that the neutral temperature be in the vicinity of the average of expectable highest and lowest rail temperatures. Should the discrepancy from that average be large, at low temperatures rail ruptures, at high temperatures rail buckling may occur.
In a given cross-section of a rail, stresses may be present even at neutral temperature, resulting from production technology (e.g. due to uneven quenching rate) or installation activities (bending of rails). Characteristic of these stresses is that their resultant referred to the entire cross-sectional area is zero.
For a given cross-sectional area the neutral temperature can be determined from the resultant of stresses arising in the given cross-section.
Non-destructive methods are based on the measurement of a physical characteristic correlated with the stress state of the section of material investigated. A common feature of all such methods is that the correlation is always associated with the actual stress state, and is dependent on the structure of the material concerned (chemical composition, texture, etc.)
The texture of metal and its residual stress depend on the applied manufacturing technology, while the distribution of residual stress may be subject to considerable changes in the course of service--especially during the early years of operation--in a way depending on the location of rails within the track and on the duty imposed on them.
One of the known non-destructive methods is based on the stress-dependence of the level of magnetic Barkhausen noise (Pashley, R. L.: Barkhausen effect--an indication of stress, Materials Evaluation, Vol. 28, No. 7, pp. 157 to 161, 1970).
Magnetic reversals taking place in ferromagnetic materials generate high-frequency electromagnetic and acoustic response signals termed Magnetic Barkhausen Noise. The level of noise depends on the structure of material and its stress state. The stress test performed by measuring the Barkhausen noise is based on the correlation existing between the level of Barkhausen noise and the stress state of the material. Application of Barkhausen noise to stress-state measurements is described in Patent Specification U.S. Pat. No. 4,634,976 and in an international patent application No. PCT/US89/01539. Common feature of the two proposed methods is that the correlation existing between noise level and stress state of a material is determined on a test piece taken from the original component or on a metal sample having a structure identical with the original by means of performing a so-called calibrating measurement.
For measuring the Barkhausen noise, the magnetic polarity of the material is periodically reversed, generally by applying an energizing field of sinusoidal or triangular shape alternating at a frequency between 10 to 100 Hz, causing emission of a high-frequency electromagnetic response signal. The response signal is detected generally by a measuring coil, typically in a frequency band ranging from a few 100 Hz up to a few 100 kHz.
For the measurement of magnetic Barkhausen noise devices of several types are available (e.g. STRESSCAN type device offered by the US firm AST. or STRESSTEST type of the Hungarian firm METALELEKTRO).
A further non-destructive method is disclosed in U.S. Pat. No. 4,405,160 according to which a specimen is magnetically excited at different points and the acoustic Barkhausen noise induced thereby is detected. By applying numerous different both positive and negative (compressive and tensile) loadings a calibration diagram can be drawn and using this calibration diagram the stress can be defined at each individual point.
This known method can be used for evaluation of the loading of a beam under laboratory conditions only. The reason for this lies in that the local maximum of the dependence of the acoustic Barkhausen noise versus stress is at the zero value of the stress, i.e. two, a positive and a negative, stress values belong to the same value of the noise. In order to be able to differentiate the (positive or negative) sense of the stress each measuring point has to be exposed both to positive and negative as well as zero stress. So as to expose each measuring point to zero stress an additional loading has to be applied onto the beam which causes a stress greater than the sum of residual stress caused by manufacture and the stress caused by ordinary loading.
Applying an additional load on a built-in beam is not allowable as being dangerous on one hand, nor can it be practically carried out on the other.
Detecting acoustic Barkhausen noise is influenced by a number of conditions, e.g. the geometric of the specimen, the resonances and reflections inside the specimen and depending on the geometry, surface roughness, contamination and corrosion of the specimen, and acoustic coupling of the sensor for detecting the Barkhausen noise to the specimen. These influences can in practice not be taken into consideration if a built-in beam is tested.
Because of the above conditions, this known method cannot be used to determine the loading of a built-in beam.